Carbon nanotube (CNT) field effect transistors (FETs) are promising candidates for future radio frequency (RF) applications because CNT FETs simultaneously offer high speed, high linearity, low power, and low noise attributes due to their unique one-dimensional semiconductor characteristics.
When grown using a standard high-temperature chemical vapor deposition (CVD) based process, one third of the nucleating CNTs are metallic nanotubes whereas two thirds are semi-conducting nanotubes. Because the metallic nanotubes have conductance without having any transconductance, the metallic nanotubes are undesirable from a device standpoint, and should be removed. The standard method of removing these metallic nanotubes is to place or grow the CNTs on a thin oxide coating on a conductive wafer. In the current technology, this process includes two parts. First, CNTs are grown in dense, aligned arrays on ST-cut quartz wafers using a standard, high temperature CVD growth process. Second, the CNTs are taken from their growth substrate and transferred to the conductive wafer using a complicated and messy CNT transfer process. The conductive wafer is then used as a global backgate to deplete the semi-conducting nanotubes before passing a large current through the metallic nanotubes. In an oxygen environment, this high current will oxidize the metallic nanotubes, leaving behind most of the depleted semi-conducting nanotubes.
Because CNT transfer is difficult, messy, and of questionable manufacturability, a method to avoid the CNT transfer process is preferred.